An assembled battery is mounted in a hybrid vehicle or an electric vehicle. The assembled battery includes a plurality of rechargeable batteries, as unit batteries, connected in series to each other. In such an assembled battery, it is necessary to detect a voltage of each of the unit batteries so as to protect and manage each of the unit batteries. However, in the assembled battery used for the hybrid vehicle or the electric vehicle, the number of series connection of the unit batteries is very large. Therefore, the potential of the unit batteries increases toward a higher potential position. That is, the unit battery connected at a higher potential position in the assembled battery has a potential higher than the unit battery connected at a lower potential position in the assembled battery. Therefore, a higher voltage is applied to the voltage detecting device of the unit battery.
JP 2012-118003 A, which corresponds to US 2012/0139545 A1, discloses a voltage detecting device for an assembled battery. In the voltage detecting device of JP 2012-118003 A, an operation amplifier and a switch are provided by a low withstand voltage transistor, without using a high withstand voltage transistor. The voltage detecting device includes a first capacitor for each of the unit batteries. A first switch is disposed between a first end of the first capacitor and a high potential terminal of the unit battery, and a second switch is disposed between the first end of the first capacitor and a low potential terminal of the unit battery. A third switch is disposed between a second end of the first capacitor and an inverted input terminal of the operation amplifier. A second capacitor and a fourth switch are connected in parallel with each other, between an input terminal of the operation amplifier and an output terminal of the operation amplifier.
In the voltage detecting device, one of a plurality of unit batteries is selected as a target of voltage detection in a predetermined order. The first switch, the third switch and the fourth are turned on to accumulate electric charge of the unit battery selected as the voltage detection target in the first capacitor. Thereafter, the first switch and the fourth switch are turned off and the second switch is turned on to detect the voltage of the selected unit battery.